High voltage transformer packages of the prior art are characterized by metal cases formed of a high thermal conductive metal such as aluminum or copper to effect rapid and efficient heat dissipation of the heat evolved during transformer operation under electrical load. Where the cases have been formed of copper or the like, the walls have been joined together and to the cover, for instance, by soldering at their abutting edges. However, soldered copper cases have normally suffered from leaks due to pressures generated internally during use, and the use of copper, aluminum and other soft but highly conductive metals inherently results in a case which lacks sufficient structural strength to permit the vacuum filling of the cases with dielectric liquid within which the transformer coils are immersed. Where the transformer package involves a cast housing or case, the cast housing while being adequately leak proof and of adequate structural strength to permit vacuum filling, is quite heavy in construction and with respect to the soldered copper or aluminum cases they are not designed to be readily reproducible; they lack operational reliability and are not vented for safety.